Seriously... why does Nick want this baby so much? Not only want the baby, but want Will 100% out of the baby's life? I really hope they give us a good backstory on his obsession with another mans child, its a little weird to me the lengths he is going to

It is, but in this situation, he's responding to Rafe saying that Sami is trying to get Will to get full custody -- i.e. getting Gabi and Nick 100% out of the baby's life. I'm glad they used that for set-up because it provides a reason for Nick and Gabi to get defensive. And, it's not even really about Will, it's about his crazy mother.